Loudspeakers can generally be grouped into three classes of device, namely electrostatic (coil and magnet), piezoelectric and capacitative. Electromagnetic loudspeakers are used in many applications, such as hi-fi systems, radios, televisions and computers. They generate high quality sound and are cheap to produce and are well established, however they suffer from the fact that they are relatively bulky and heavy, and have limited control over the directionality of the generated sound. Whilst electromagnetic loudspeakers can be made which cover the range of frequency from sub-audio (10 Hz) to the top of the hearing range (20 kHz), it is usual for two or three separate loudspeakers to be used together to span the whole audio frequency range if high fidelity reproduction is required.
Loudspeakers based on piezoelectric principles are currently of considerable interest as they can be used to produce flat loudspeakers which are relatively thin (several mm), and are particularly advantageous where space is at a premium, for example in aircraft or in cars. However such loudspeakers can be relatively expensive to produce and are inflexible, limiting their flexibility of use.
Other Piezoelectric sound sources (with very low sound quality) are produced, and an example of this class of piezoelectric sound source is the “unimorph” used in singing Christmas cards.
Recently, flat panel loudspeakers have appeared on the market, which have a distributed mode source, offering better directionality that has been possible with previous loudspeaker arrangements. These are flat, but still require an excitation mechanism (generally an electromagnetic arrangement, but variants using piezoelectric excitation are possible). There is a maximum size of this class of transducer, meaning that large area sources (desired for some applications) must be made from an array of these devices, limiting the directionality of the source.
Electrostatic loudspeakers are often considered to give the highest quality audio reproduction. Generally such loudspeakers use an electrically conducting thin membrane between two electrode planes. During operation the membrane is electrostatically charged with a high (DC) polarising voltage. If an (AC) audio signal is applied between the two electrode planes a varying electric field will be established which will have the effect of causing the diaphragm to move back and forth at the frequency of this voltage generating sound. However such loudspeakers use very high voltages (1000V and above) and require a bulky enclosure. They also have reduced low-frequency (bass) response.
WO02/19764 discloses an electrostatic audio loudspeaker comprising a multi-layer panel incorporating an electrically insulating middle layer sandwiched between first and second electrically conducting outer layers, at least one of the layers having a profiled surface where it contacts the surface of another of the layers, and signal means for applying an alternating electrical voltage across the first and second layers to initiate vibration due to variation of the electrostatic forces acting between the layers. Such a loudspeaker operates satisfactorily in many applications, but does not provide the best quality sound reproduction, or the loudest output for a given drive voltage.
It is an object of the present invention to provide a novel electrostatic loudspeaker which is capable of better quality and louder sound reproduction than that disclosed in WO02/19764, and which is capable of being used in a variety of applications, and particularly in applications where space or weight is at a premium, or where a large area or directional sound source is desirable.